1. Field of the Invention
The present invention relates generally to a channel assignment method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a method of assigning a P-RACH (Physical Random Access Channel) for data transmission on an RACH (Random Access Channel).
2. Description of the Related Art
The demand for a mobile communication system supporting the transmission of data and pictures as well as a typical voice service is growing along with the rapid development of mobile communication technology. This type of mobile communication system is known as the future mobile communication system. The future mobile communication system is generally implemented in CDMA. CDMA systems are divided into synchronous ones and asynchronous ones. The U.S. employs a synchronous CDMA system whereas Europe and Japan employ an asynchronous CDMA system. Therefore, different standardization tasks are in work in the U.S. and Europe. The European future mobile communication system is known as UMTS (Universal Mobile Telecommunication System).
The production of globally applicable technical specifications is being undertaken with 3GPP (3rd Generation Partnership Project) for the UMTS system and an NB-TDD (Narrow Band Time Division Duplexing) system where base stations are synchronized to one another. The 3GPP organization has submitted technical reports about the NB-TDD system. The contents of the technical reports are incorporated herein by reference and include a random access procedure as summarized below.
In the NB-TDD CDMA communication system, the random access procedure is performed in three stages: synchronization to a UTRAN (UMTS Terrestrial Radio Access Network) by a time division frame and acquisition of UTRAN information, acquisition of a P-RACH, and transmission of RACH data on the P-RACH.
Before a description of the random access procedure, the structure of a (radio) frame is described. The (radio) frame is 10 ms in duration and includes two sub-frames. Each sub-frame is composed of seven normal time slots, downlink pilot time slot (DwPTS), and uplink pilot time slot (UpPTS).
(1) Synchronization to UTRAN and acquisition of UTRAN information: A UE (User Equipment) synchronizes its timing to the UTRAN using a 64-bit SYNC code received in a downlink pilot time slot. There are 32 SYNC codes and each UTRAN is assigned to one SYNC code. The UE identifies a SYNC code assigned to a specific UTRAN using a matched filter. After the identification, the UE can be synchronized to the base station. If the UE succeeds in synchronization, it can obtain information about a P-RACH and an F-PACH (Fast Physical Access Channel) for use in the random access procedure based on UTRAN information, which is broadcast periodically from the UTRAN. The P-RACH and FPACH information includes codes assigned to the P-RACH and the FPACH, a spreading factor (SF), midambles, and time slots. The UE determines the one-to-one matching relationship between the FPACH and the P-RACH from the UTRAN information.
(2) Acquisition of P-RACH: Each SYNC code is assigned eight SYNC1 codes. The UE selects one of the eight SYNC1 codes assigned to the SYNC code identified in stage 1 and transmits the selected SYNC1 code in an uplink pilot time slot. The UTRAN detects the arrival time and power of the SYNC1 code, calculates time and power variations, and transmits the variations to the UE within four or less sub-frames on a corresponding FPACH. The corresponding FPACH refers to an FPACH assigned to the selected SYNC1 code. The relationship between the SYNC1 code and the FPACH is also included in the UTRAN information of stage 1. The FPACH includes information about the type of the SYNC1 code and a sub-frame in which the SYNC1 code was received. After transmitting the SYNC1 code, the UE monitors FPACHs in the four sub-frames and if it receives an FPACH for the UE, determines that the UTRAN has acknowledged the SYNC1 code sent by the UE. Then, the UE is authorized to use a P-RACH corresponding to the FPACH.
(3) Transmission of RACH data: The UE is assigned to the P-RACH two sub-frames after the FPACH. The UE transmits RACH data on the P-RACH.
In the above random access procedure, SYNC1 codes correspond one to one to FPACHs and P-RACHs. Therefore, if less than eight FPACHs or P-RACHs are defined in one sub-frame, less than eight SYNC1 codes may be used. This implies that less SYNC1 codes are available to UEs and as a result, channel collision is likely to increase. One sub-frame is a unit time set in the NB-TDD CDMA communication system, 5 ms. If more SYNC1 codes than FPACHs or P-RACHs per sub-frame are used, P-RACH resources are dissipated and a connection time delay is prolonged due to the one-to-one matching relationship between SYNC1 codes and FPACHs/P-RACHs.